Conventional ink-jet printing systems use various methods to cause ink droplets to be directed toward recording media. Well known ink-jet printing devices include thermal, piezoelectric, and acoustic ink jet print head technologies. All of these ink-jet technologies produce roughly spherical ink droplets having a 15-100 μm diameter directed toward recording media at approximately 4 meters per second. Located within these print heads are ejecting transducers or actuators, which produce the ink droplets. These transducers are typically controlled by a printer controller, or conventional minicomputer, such as a microprocessor.
A typical printer controller will activate a plurality of transducers or actuators in relation to movement of recording media relative to an associated plurality of print heads. By controlling activation of transducers or actuators and recording media movement, a printer controller should theoretically cause produced ink droplets to impact recording media in a predetermined way, for the purpose of forming a desired or preselected image on the recording media. An ideal droplet-on-demand type print head will produce ink droplets precisely directed toward recording media, generally in a direction perpendicular thereto. However, in practice, a number of ink droplets, for various reasons, are not directed exactly perpendicularly to the recording media; and, ink droplets that deviate from a desired trajectory, and which result in misdirected droplets impacting recording media at locations not anticipated by a controller of a printer, are problematic. As a result, the misdirected droplets generally negatively affect the quality of a printed image—typically by impacting the recording media in undesired locations.
To correct misdirected ink jet trajectories, for instance, U.S. Pat. Nos. 4,386,358 and 4,379,301 disclose methods for electrostatically deflecting electrically charged ink droplets ejected from ink jet print heads. Briefly summarizing methods disclosed in these patents, charges placed on electrodes on the print heads are “controlled,” to steer charged ink droplets in desired directions to compensate for known print head movement. By electrostatically steering the charged ink droplets thusly, the methods disclosed in these patents compensate for ink droplet misdirection caused by the known print head movement, when an ink droplet is ejected. However, the electrostatic deflection method disclosed in these patents does not compensate for unanticipated or unpredictable factors, which can affect ink droplet trajectories.
To solve the problem (noted in the preceding paragraph) U.S. Pat. No. 6,079,814 discloses a droplet-on-demand ink jet printer that makes use of an electrostatic phenomenon known as “tacking,” which is simply the attachment, resulting from electrostatic attraction, of one item or article to another. In the application of this well-known electrostatic principle, U.S. Pat. No. 6,079,814 discloses tacking recording media, e.g., paper, in order to achieve precise attachment of an aligned piece of recording media onto a dielectric surface of a transport belt, for achieving assurance of precise motion of the recording media relative to the print heads, for precise ink droplet placement on the recording media. To summarize, a transport belt is electrostatically charged with a charge of one polarity, so that the resulting electrostatic charge precisely holds the recording media in a precisely aligned position on the transport belt after the media is fed thereon and concurrently induces a charge of opposite polarity on the ink droplets ejected by the print head, for accelerating the ink droplets toward the recording media.
As a refinement, or perhaps reversal, of the tacking phenomena discussed above in U.S. Pat. No. 6,079,814, U.S. Pat. No. 8,293,338 describes and discloses a process whereby print media sheets are moved downwardly past a so-called “de-tacking unit,” designed to reverse the electrostatic charge on the print media, in order to allow transfer of the print media from a first endless belt to a second endless belt. U.S. Pat. No. 8,293,338 describes and discloses the second belt as passing over a porous stationary platen. U.S. Pat. No. 8,293,338 further states that the platen is connected through a conduit to a vacuum pump which, via the platen porosity and first belt, causes the sheet stock to adhere to the platen and remain vertically positioned thereon.
As perhaps still another refinement of the tacking phenomena described above (in relation to U.S. Pat. No. 6,079,814), U.S. Pat. No. 8,408,539—which is directed to a sheet hold down and transport apparatus—discloses and describes inboard and outboard tacking rollers that are in operative communication with a high-voltage power source, wherein the tacking rollers deposit a static charge on an upper surface of certain edges of a media sheet. U.S. Pat. No. 8,408,539 further discloses that a transport belt is preferably formed of a nonconductive material; and that the charged surface of the sheet edges are attracted to the belt. U.S. Pat. No. 8,408,539 further discloses that the tacking rollers are biased to a potential sufficiently high to generate air breakdown adjacent to a nip formed by the tacking rollers and the belt, and that as a sheet enters the nip, the air breakdown will deposit net charge onto the top of the sheet along its inboard and outboard edges, thereby holding the sheet edges flat to the belt. This patent further discloses that medial portions of the belt, between the tacking rollers constitutes an image zone which aligns with a print head, that the portion of the sheet of media lying in the image zone will receive the image, and that positioning the tacking rollers on the sheet edges and outside the image zone, the image zone remains substantially free of electrostatic charges.
Still another solution to the problems caused by misdirected ink jet droplets impacting recording media is described in U.S. Pat. No. 7,204,584 which discloses and describes a transfer belt apparatus, a system and various methods, all of which are directed to preventing image blooming, where the term “image blooming” is understood to mean that a printed image is wider, occasionally much wider, than desired and may have indistinct edge margins, all of which are problematic. Accordingly, to prevent image blooming, U.S. Pat. No. 7,204,584 discloses that an ink jet printing apparatus may include a grounded print head, a counter-electrode opposite the grounded print head, and a bi-layer transfer belt located between print heads and the counter-electrode and at least partly supported by at least two transfer bias rollers. U.S. Pat. No. 7,204,584 also discloses and describes a particular method of operation that may include applying a predetermined voltage between a print head and a counter-electrode to accelerate ink drops ejected from a print head toward a transfer belt, for removing charge on the belt.
To further refine the “image blooming” problem, described above, U.S. Pat. No. 8,142,010 discloses and describes a transporting belt for inkjet use, where the belt is characterized by a seamless belt shape having at least one layer comprising at least one of a polyamide resin, a polyester resin, and a polyimide resin, as the resin component and a conductive filler, and having a volume resistivity of about 1010 to 1014 ohm-centimeters (Ω·cm).
As yet another solution to the problems caused by misdirected ink jet droplets impacting recording media, certain embodiments of a system to reduce electrostatic fields underneath print heads in a direct marking printing system are disclosed and described in U.S. Pat. No. 8,947,482. One such system that is disclosed in U.S. Pat. No. 8,947,482 includes one or more print heads for depositing ink onto a media substrate; a media transport for moving the media substrate along a media path past the one or more print heads; a conductive platen contacting the media transport belt; an electrostatic field reducer that includes an alternating current charge device positioned upstream of the one or more print heads; and one or more electrically biased electrodes in registration with the ink deposition areas of the one or more print heads. U.S. Pat. No. 8,947,482 states that the media transport includes a media transport belt which, when media is on the transport belt, can generate an electrostatic field, to cause printing defects. U.S. Pat. No. 8,947,482 states that the electrostatic field reducer along with the electrodes reduce the electrostatic field on the surface of the media and thereby reduce printing defects.
Still another solution to problems caused by misdirected ink jet droplets impacting recording media are disclosed and described in U.S. Pat. No. 9,114,609—which is directed to an inkjet printer system that includes an electrode located either in a print head or in an image receiving member, where the image receiving member is operatively connected to a waveform generator. During operation of the system, a controller operates the waveform generator to generate an electrostatic field between the print head and the image receiving member during normal operation of inkjets in the print head to eject ink drops. In particular, the controller operates the waveform generator to reduce an amplitude of the electrostatic field while the ink drops travel toward the image receiving member during a time when satellite ink drops can be formed from ejected ink drops. The controller also subsequently operates the waveform generator to generate the electrostatic field while the ink drops are in flight after formation of the satellites, to accelerate the ink drops and satellites towards the image receiving member.
As yet another solution to problems caused by misdirected ink jet droplets impacting recording media, U.S. Pat. No. 9,132,673 discloses a semi-conductive media transport system used in conjunction with an ink jet printing system. Since the purpose of “an invention” is often to solve “a problem,” a problem the U.S. Pat. No. 9,132,673 inventors focused their efforts on may be stated thusly: In order to ensure good print quality in direct-to-paper (DTP) ink-jet printing systems, the media must be held extremely flat in the print zone. The belt itself must be held flat against a platen; and, once accurate registration of the substrate media is achieved, the media must not be allowed to move out of registration as it is delivered to the print zone.
Because contemporary systems, of that time, transferred media by means of laterally spaced-apart drive rollers located in registration nip assemblies, U.S. Pat. No. 9,132,673 inventors noted that rollers of that era do not hold the media flat, and therefore can subject the media to misalignment. These inventors noted that media acquisition by the belt can be by electrostatic tacking, and they further noted that such electrostatic tacking has the advantages of holding the media flat, and eliminating registration shift. These inventors further noted that a vacuum on the platen may be used to further ensure flatness. A problem these inventors noted arises in that friction-induced tribo-electric charges between a belt and platen (and elsewhere) generate undesirable electrostatic fields in the ink ejection area which may adversely affect print quality. These inventors noted that use of a conductive belt will circumvent this but can make it difficult to achieve desirable low, controlled fields between the media and a print head over a wide range of media properties. Based on these problems, the U.S. Pat. No. 9,132,673 inventors disclosed and described a system, where a belt is held flat and caused to slide across an electrically conductive platen, which could result in build-up of electrostatic charges on the belt, were it not for the fact the belt is semi-conductive, specifically to prevent charge build-up.
Thus, their belt was provided with an effective surface resistivity between a lower limit to preclude a build-up of electrostatic charges, and an upper limit to enable electrostatic tacking of media to the belt, where effective surface resistivity limits vary depending on belt velocity, thickness, material, belt and media dielectric constant, and slot width. U.S. Pat. No. 9,132,673 also discloses a pair of charged nip rollers that tack the media substrate to the belt.
However, U.S. Pat. No. 9,132,673 did not solve a problem associated with the contamination of printhead faceplates, resulting from the “misting” of the printhead faceplates.
While these various approaches to solving the many problems that are associated with conventional ink-jet printing systems have resulted in marked improvements being made to assorted ink-jet printing systems throughout the years, high-speed printing operations along with current demand for the highest levels of print quality are unrelenting. Thus, the message is really quite simple. To retain customer loyalty, superior image quality must be maintained.